Backporting to Lua 5.1

[ggcrunchy]

Hi.

I'm attempting what's described in the title. :)

I still have a few 5.3-specific functions to shim, but nothing that sounds too awful.

Am now trying to deal with the integers.

From what I can see, I can find all of the entity usage with luaL_checkinteger() and lua_pushinteger(), and then there are the opcodes in meta.c. Anything else to be aware of? (In flecs proper I suppose there might be the DSLs to consider.)

It looks like there's an integer vs. number case in the serialization logic, so a lua_Number (as suggested by this comment) won't be quite enough, but I'm thinking a light userdata would work on 64-bit targets, and maybe some hybrid number / full userdata approach otherwise, with some helper functions around them.

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(Maybe later I could adopt the LNUM patch, but not for the time being.)